Synthetic aperture radar imaging apparatus and methods for moving targets

ABSTRACT

A synthetic aperture radar (SAR) system may employ SAR imaging to advantageously estimate or monitor a transit characteristic (e.g., velocity, acceleration) of a vehicle, for example a ground based vehicle or water based vehicle. A dual-beam SAR antenna illuminate a moving target with a first radar beam and a second radar beam at an angular offset relative to the first radar beam. Pulses may be transmitted and backscattered energy received simultaneously by the SAR transceiver via the first and second radar beams. A SAR data processor may generate a first image from the first radar beam and a second image from the second radar beam, co-registering the first and second images, comparing the location of the moving target in the first and second images, and estimate a velocity of the moving target based at least in part on the angular offset.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No. 16/616,364 filed Nov. 22, 2019, which is National Stage Entry of PCT/US2018/034146 filed May 23, 2018, and claims priority from Provisional Application 62/510,191 filed May 23, 2017, the contents which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present application relates generally to synthetic aperture radar (SAR) and, more particularly, to operating modes suitable for estimating the velocity of a moving target.

BACKGROUND Description of the Related Art

A synthetic aperture radar (SAR) is an imaging radar. The SAR exploits the relative motion of the radar and a target of interest to obtain high azimuthal resolution. The SAR is typically flown on an aircraft, a spacecraft, unmanned aerial vehicle (UAV) such as a drone, or another suitable platform. The target of interest is typically on the ground (e.g. on land, water, ice or snow), and can be a point target or a distributed target. The SAR can be a component of a SAR imaging system, the system also including at least one of data processing and data distribution components.

In conventional operation of a SAR imaging system, the system is tasked to obtain images of a target or a swath. Data is collected on-board the platform. In the case of a spaceborne SAR, the data is collected on-board the spacecraft, and either processed on-board the spacecraft and downlinked to the ground, or downlinked and processed on the ground to generate the images. The images are distributed to the user, typically via a network. In some implementations, the main elements of a space-borne SAR platform can include:

Satellite Platform: includes the following subsystems and units: Structure, Power, On-board Data Handling, a Payload Data Handling Unit, Telemetry and Telecommands;

-   -   Communications (TT&C), X-Band High-rate Downlink, Attitude and         Orbit Control subsystem, Thermal Control, and Propulsion;     -   SAR Instrument; and/or     -   A SAR Processing Unit: performs onboard SAR data processing.

BRIEF SUMMARY

Some embodiments of SAR systems can incorporate the following advanced SAR features into a single SAR instrument:

-   -   a shared aperture;     -   multi-aperture (e.g., in one implementation, six apertures for a         SAR operating at X-band, three apertures for a SAR operating at         L-band);     -   digital beam-forming (with multiple beams in elevation and         azimuth);     -   quad-polarization and compact polarization; and/or     -   modular multi-aperture technology with digital interfaces of SAR         Data.

In the case of a dual-band SAR, the SAR can have simultaneous dual-frequency capability (e.g., L-band and X-band).

SAR systems can include multiple digital and RF components. In some implementations, a SAR system includes a SAR antenna, sensor electronics, and Transmit Receive Modules (TRMs) mounted on an antenna panel.

A SAR Processing Unit (SPU) can be part of an On-Board Data Handling subsystem. The SPU may house processing boards, power boards, cabling, and an associated backplane. Each processing board in the SPU can include multiple ultra-high performance FPGA boards, for example, that can perform real-time processing tasks. The processing functions performed by the SPU can include the following:

-   -   on-board SAR Data Processing;     -   target detection; and/or     -   compression/packetization/encryption/forward error correction         encoding for communications links.

A method of operation of a synthetic aperture radar (SAR) system to estimate the velocity of a moving target may be summarized as including a dual-beam SAR antenna, a SAR transceiver and a SAR data processor, the SAR transceiver communicatively coupled to the dual-beam SAR antenna and to the SAR data processor; the method including directing a first radar beam to illuminate the moving target in a region on a surface of the Earth by the dual-beam SAR antenna; directing a second radar beam to illuminate the moving target by the dual-beam SAR antenna, the second radar beam at an angular offset relative to the first radar beam; transmitting pulses and receiving backscattered energy simultaneously via the first and second radar beams by the SAR transceiver; generating, by the SAR data processor, a first image from the first radar beam and a second image from the second radar beam; co-registering the first and the second images by the SAR data processor; comparing, by the SAR data processor, the location of the moving target in the first image and the second image; and estimating, by the SAR data processor, a velocity of the moving target based at least in part on the angular offset. Directing a first radar beam to illuminate the moving target may include directing a forward-looking radar beam to illuminate the moving target, and directing a second radar beam to illuminate the moving target may include directing an aft-looking radar beam to illuminate the moving target. Directing a first radar beam to illuminate the moving target may include directing a radar beam comprising a main lobe of an antenna beam pattern to illuminate the moving target, and directing a second radar beam to illuminate the moving target may include directing a radar beam comprising a grating sidelobe of the antenna beam pattern to illuminate the moving target. Directing a radar beam including a main lobe of an antenna beam pattern to illuminate the moving target and directing a radar beam including a grating sidelobe of the antenna beam pattern to illuminate the moving target may include applying a phase ramp across an aperture of the dual-beam SAR antenna. Applying a phase ramp across an aperture of the dual-beam SAR antenna may include causing a magnitude of the grating sidelobe of the antenna beam pattern to be approximately the same as a magnitude of the main lobe of the antenna beam pattern.

The method may further include forming two or more elevation beams; and generating a SAR image with multi-looking in range. Transmitting pulses and receiving backscattered energy simultaneously via the first and second radar beams by the SAR transceiver may include transmitting pulses and receiving backscattered energy in a ScanSAR imaging mode.

A synthetic aperture radar (SAR) system may be summarized as including a SAR platform including at least one SAR antenna; and at least one processor; and at least one nontransitory processor-readable medium communicatively coupled to the at least one processor which stores at least one of processor-executable instructions or data which, when executed by the at least one processor, may cause the at least one processor to perform any of the above methods.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not necessarily drawn to scale, and some of these elements may be arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn, are not necessarily intended to convey any information regarding the actual shape of the particular elements, and may have been solely selected for ease of recognition in the drawings.

FIG. 1 is a graph of an example dual-beam azimuth antenna pattern at L-band, in accordance with the systems and methods of the present application.

FIG. 2 is a graph of an example dual-beam azimuth antenna pattern at X-band, in accordance with the systems and methods of the present application.

FIG. 3 is a graph of relative performance of single-beam and dual-beam SAR systems, in accordance with the systems and methods of the present application.

FIG. 4 is a block diagram of an example SAR system, in accordance with the systems and methods of the present application.

FIG. 5 is an isometric view of a SAR antenna in the form of a planar phased array antenna assembly, in accordance with the systems and methods described in the present application.

DETAILED DESCRIPTION

Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.”

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its broadest sense, that is as meaning “and/or” unless the content clearly dictates otherwise.

The Abstract of the Disclosure provided herein is for convenience only and does not interpret the scope or meaning of the embodiments.

Different imaging modes for a SAR are described below. Particular emphasis is given to wide-swath and ultra-high resolution modes, surveillance modes that can be used for target detection, and experimental modes that include very small target detection and target velocity estimation modes. The surveillance and experimental modes can be used, in particular, for maritime surveillance, for example where the targets are ships.

SAR-XL Imaging Modes—StripMap Imaging and ScanSAR

StripMap imaging mode: SAR can use a single fixed beam with a single aperture to acquire a continuous image strip.

ScanSAR imaging mode: SAR can use electronic beam steering to periodically switch within a set of adjacent beams which are later processed into a wide continuous swath at a lower resolution as compared to StripMap.

In a dual-band SAR, StripMap image modes can be available in X-, L- or simultaneous X- and L-band, and, in some implementations, in a variety of transmit and receive polarizations including quad-polarization (HH, VV, HV, and VH, where H is horizontal polarization and V is vertical polarization). In StripMap modes, the image resolution typically varies from 1.0 m to 20 m, and the swath width typically varies from 7.5 km to 50 km, depending on the specific mode. These modes can provide high image quality in terms of conventional image quality metrics, e.g., NESZ (Noise Equivalent Sigma Zero), Range Ambiguity to Signal Ratio (RASR) and Azimuth Ambiguity to Signal Ratio (AASR).

A conventional ScanSAR mode typically uses multiple beams to get a wider swath width than the StripMap modes. The swath width can vary from 100 km to 500 km depending on incidence angle, with a 30 m resolution.

SAR Imaging Modes—Surveillance Modes

The flexibility of advanced SAR systems can enable the generation of application-specific modes unavailable in conventional less-advanced SAR systems. For example, enhanced ScanSAR modes can be tailored specifically for target detection (e.g., watercraft, ship, or vehicle detection), and can provide almost uniform target detection performance across an accessible area. For example, in the case of maritime surveillance, modes can be tailored specifically for ship detection, and can provide almost uniform target detection performance with a Minimum Detectable Ship Length (MDSL) of 25 m or better across an accessible area.

In some implementations, an advanced SAR system includes a dual-band SAR, i.e., a SAR operable to generate SAR images at two different frequency bands. In some implementations of a dual-band SAR, enhanced ScanSAR modes tailored specifically for target detection can include two L-band modes and three X-band modes that collectively can provide access to a ground range swath of between 150 km off-nadir to 575 km off-nadir.

In some implementations, L-band modes can be tailored for maritime surveillance and ship detection in near-range, and can utilize HV cross-polarization for improved clutter suppression covering incidence angles of 19.7 degrees to 45.2 degrees. HV cross-polarized images can be generated by including transmitting radar pulses in horizontal (H) polarization and receiving backscattered radar pulses in vertical (V) polarization. X-band modes can take advantage of additional X-band antenna gain and wider bandwidth. In an example implementation of a dual-band XL (X-band and L-band) SAR system, X-band modes can cover incidence angles from 31.0 degrees to 55.5 degrees. Examples of various target detection modes for a dual-band XL SAR are tabulated below in Table 1. Other suitable modes can be constructed.

TABLE 1 Example Dual-Band SAR Target Detection ScanSAR Modes Characteristics Ground Ground Swath Start End Width Number of Band Pol (km) (km) (km) Beams SD Mode A L-Band HV 150 378 228 4 SD Mode B L-Band HV 200 410 210 5 SD Mode C X-Band VV 250 455 205 8 SD Mode D X-Band VV 300 533 233 8 SD Mode E X-Band VV 350 575 225 8

SAR beam modes can be tuned to suit a particular surveillance scenario. For example, modes can be tuned to suit maritime surveillance based at least in part on an understanding of the effects of different beam choices on the likely detectability of vessels in cluttered and noisy SAR imagery.

For example, a model can be built for ship detectability that may include inputs such as any one or more of the following:

-   -   frequency band (e.g., L or X);     -   polarization (HH, VV, HV or VH);     -   sea state (e.g., 3 or 5);     -   wind direction relative to beam (e.g., an angle between 0° to         90°);     -   incidence angle (e.g., from 20° to 60°);     -   K-distribution shape parameter (e.g., 4, as in RD-1);     -   effective number of independent looks (e.g., 2 or 4);     -   probability of false alarm (e.g., 10⁻⁹ or 10⁻⁶);     -   probability of detection (e.g., 90% or 80%); and/or     -   NESZ, azimuth and ground-range resolutions of the SAR beam(s) at         the given incidence angle(s).

The output of the modeling can be, for example, a Minimum Detectable Ship Length (MDSL), for which the computed probability of detection is above a threshold value, and the backscattered power in a given frequency band and polarization is above a threshold value for the probability of false alarm of K-distributed sea clutter under ocean conditions specified in the inputs to the model.

Target Velocity Estimation Approach

In one example scenario, the systems and methods described in the present application can be used for maritime surveillance. The SAR system can use a wide-swath SAR imaging mode such as a ScanSAR mode, and can process the wide-swath SAR data, on-board or on the ground, to detect moving targets (e.g., ships and other watercraft) and estimate their velocity (e.g., speed and heading). In other scenarios, the systems and methods described in the present application can be used to detect land, snow, or ice-based targets, and estimate their velocity.

To estimate the velocity of watercraft, vehicles, and other moving targets, special beams can be developed within the SAR modes identified above. An operational approach for using these special beams can include a dual-beam SAR imaging approach for measuring target motion directly (i.e., from an analysis of the dual-beam SAR images).

In some implementations, a SAR antenna consists of multiple azimuth phase centers. Each of the multiple phase centers has sensor electronics that can control the phase of signals being fed to radiating elements of the SAR antenna. A phase ramp can be applied across an antenna aperture to steer an antenna beam. The extent to which the antenna beam can be steered can be limited by the beam pattern of a single antenna phase center. As the beam is steered towards the edge of the beam pattern of the single antenna phase center, a grating lobe can appear, and the grating lobe can become larger relative to the main lobe the more the beam is steered.

An azimuth beam can be steered to an angle at which the gain of the grating lobe is approximately the same magnitude as the gain of the main lobe, for example by steering the azimuth beam by a steering angle of one half of the beam width of a single azimuth phase center, as follows:

θ=0.5×0.886×λ/A

where λ is a wavelength of illumination, and A is an azimuth dimension of an azimuth phase center.

In one example implementation, at X-band, A=1 m, and θ=0.8°, and at L-band, A=2 m, and θ9=3.0°. The grating lobe appears at the negative of this angle, and the separation between the beams is approximately twice this angle.

A SAR in dual-beam operation can transmit and receive simultaneously through both beams, albeit at half the antenna gain relative to a single-beam system. Data can be simultaneously received from both forward-looking and aft-looking beams, for example. The two beams can have an angular offset between one another. In one implementation, one beam is forward of a broadside direction relative to a ground track of the SAR and another beam is aft of the broadside direction. In one implementation, two beams are both forward of a broadside direction, one beam more forward than the other. In one implementation, two beams are both aft of a broadside direction, one beam more aft than the other. In some implementations, a first beam is in a broadside side direction and a second beam is either forward or aft of the first beam. In the present application, the most forward-looking beam of the two beams is referred to as a forward-looking beam, and the other beam of the two beams is referred to as an aft-looking beam.

A SAR processor can generate separate images from the SAR data received from each beam. When processing the SAR data from a first beam of the two beams, signals from a second beam of the two beams can appear as an interference to the first beam, but signals from the second beam will not focus at least in part because range walk in the second beam is in the opposite direction to range walk in the first beam. Range walk is an effect in which a moving target may straddle more than one range cell during a single coherent processing time interval.

Using a dual-beam approach described in the present application, an aft-looking beam can illuminate a region on the Earth's surface that overlaps at least a portion of a region illuminated by a forward-looking beam, at a later time than the at least a portion of the region was illuminated by the forward-looking beam. The images formed by the forward-looking and the aft-looking beams are offset in time from one another. In some implementations, the offset in time (also referred to in the present application as the time offset) can be several seconds. When the two images of the overlapping area are co-registered, and the locations of the same target in the overlapping area are compared, the time offset can be sufficient to provide information about the motion of a target that is detected in both images. For example, the two images can be co-registered and analyzed to determine an estimate of a velocity (e.g., speed and heading) of the target.

The systems and methods described in the present application can include forming one or more images of a target or region on the Earth's surface, for example, on land, water, snow or ice. Targets can include point targets and distributed targets. Targets can include stationary targets and moving targets. Targets can include vehicles, ships, submarines, and other man-made objects.

FIG. 1 is a graph 100 of an example dual-beam azimuth antenna pattern 102 at L-band, in accordance with the systems and methods of the present application.

At a slant range of approximately 500 km, an angular offset between the two beams of ±3.0° can correspond to a distance of approximately ±26 km. The SAR antenna on a spaceborne platform in a low Earth orbit can take approximately 7.2 s to travel that distance, during which time a target travelling at a speed of 10 m/s can travel approximately 72 m. The time offset can provide sufficient time to measure a speed and heading of the target.

An accuracy of a velocity estimate (denoted by σ_(v)), derived from a relative position shift in the SAR images, as a function of a positioning error σ_(m), and a time offset T between the measurements, can be expressed as follows:

σ_(v)=√{square root over (2)}×σ_(m) /T

Accuracy of the measurement can depend, at least in part, on a SAR resolution, which can, in turn, depend on characteristics of a ScanSAR mode used to acquire the SAR data. For example, accuracy of the measurement can depend on the number of elevation beams used, and on use of multi-looking. In some implementations, a higher accuracy and improved results can be achieved by using range multi-looking.

Typically, a SAR with resolution (ρ) can be lead to a positioning error of:

σ_(m)=ρ/√{square root over (12)}

For example, an L-band ScanSAR mode with a resolution of 10 m can lead to a positioning error of approximately 2.9 m, and a velocity estimation error of approximately 0.6 m/s. The positioning error can be in the along-track and the across-track direction.

In some implementations, the positioning error can be improved. For example, a zero-padded Fast Fourier Transform (FFT) can be used, in some cases in combination with other processing methods, to improve the accuracy with which a target can be located by finding the location of a scattering center within a resolution cell.

FIG. 2 is a graph 200 of an example dual-beam azimuth antenna pattern 202 at X-band, in accordance with the systems and methods of the present application.

At a slant range of approximately 500 km, an angular offset between the two beams of ±0.8° can correspond to a distance of approximately ±7 km. The SAR antenna on a spaceborne platform in a low Earth orbit can take approximately 1.9 s to travel that distance, during which time a 10 m/s target can travel approximately 19 m.

An X-band ScanSAR mode with a resolution of 10 m can lead to a positioning error of approximately 2.9 m, and a velocity estimation error of approximately 2.2 m/s. In one implementation, the radial component of the velocity estimate is improved by fusing a velocity estimate derived using the technology described above with a radial velocity estimate derived from a conventional method such as Along-Track Interferometry (ATI).

While use of a dual-beam system, as described above, can result in a loss in antenna gain of 3 dB on both transmit and receive, the loss applies equally to both target and clutter. So, in the case of a clutter limited performance, there is no overall change to performance resulting from the 3 dB loss in antenna gain.

Though use of a dual-beam system, as described above, can increase a clutter level by a factor of two causing a 3 dB degradation, the degradation can be offset by other factors. For example, by using both beams of the dual-beam system to detect a target, a dual-beam system can have an improved false alarm rate. The false alarm can be improved by the dual-beam approach over conventional approaches by a factor of a square root. For example, if the single-beam false alarm rate is 10⁻¹⁰, the dual-beam false alarm rate can be of the order of 10⁻⁵ to achieve approximately the same overall false target rate. Similarly, the probability of detection for the dual-beam can be 0.95 to achieve a two-out-of-two detection probability of 0.9.

FIG. 3 is a graph 300 of relative performance of single-beam and dual-beam SAR systems, according to the present disclosure. Graph 300 includes a single-beam plot 302 of required radar cross-section (RCS) for a probability of detection of 0.9 versus ground range, and a dual-beam plot 304 of required radar cross-section (RCS) for a probability of detection of 0.9 versus ground range. Graph 300 can be generated by simulation, for example.

As shown in FIG. 3 , an overall loss in detection of a dual-beam system (also referred to in the present application as a dual azimuth beam system) compared to a single-beam system (also referred to in the present application as a single azimuth beam system) can be of the order of 2 dB, in a clutter-limited case. The loss in detection can increase the Minimum Detectable Ship Length (MDSL) by approximately 25%. For example, in the case of a ScanSAR mode with a MDSL capability of 16 m with a single azimuth beam, performance can be degraded to a MDSL of 20 m with a dual azimuth beam. A benefit of a dual azimuth beam system is that it can provide a direct measurement of both target speed and heading.

Though in a noise-limited case a loss between single-beam and dual-beam can be of the order 4 dB, detection performance for the noise-limited case can be better than for a clutter-limited case, and the additional performance margin afforded by the better detection performance can generally be able to absorb the loss.

One approach to avoiding a performance penalty that could, for example, result in an increase in the MDSL by 25%, is to adjust the characteristics of the SAR beams to maintain the MDSL performance capability at the expense of SAR swath width.

Starting from the ScanSAR modes described in Table 1, a new set of beams was developed to maintain an MDSL of 25 m while reducing the SAR swath width to 150 km for each of the ScanSAR modes. The new set of beams is referred to in the present application as dual-azimuth target detection Scan SAR modes, and are described in Table 2 (below). An estimated velocity error for the L-band modes is 0.6 m/s (across-track and along-track). An estimated velocity error for the X-band modes is 2.2 m/s (across-track and along-track).

TABLE 2 Dual-Azimuth Target Detection ScanSAR Mode Characteristics Ground Ground Swath Start End Width Number of Band Pol (km) (km) (km) Beams SD Mode A L-Band HV 200 350 150 3 SD Mode B L-Band HV 250 400 150 3 SD Mode C X-Band VV 300 450 150 5 SD Mode D X-Band VV 350 500 150 5 SD Mode E X-Band VV 425 575 150 5

While the systems and methods described in the present application are particularly suited to maritime surveillance and ship detection and velocity estimation, the systems and methods described in the present application can apply to SAR surveillance more generally, including surveillance of water, land, snow, and ice, and to moving target detection of watercraft, vehicles, and other moving targets.

A method for estimating the velocity of a moving target according to the present disclosure can include the following acts:

-   a) directing a first radar beam to illuminate a moving target in a     region on a surface of the Earth by a dual-beam SAR antenna; -   b) directing a second radar beam to illuminate the moving target by     the dual-beam SAR antenna where the second radar beam is at an     angular offset from the first radar beam; -   c) transmitting radar pulses and receiving backscattered energy     simultaneously via the first and second radar beams by the SAR     transceiver; -   d) generating , by the SAR data processor, a first image from the     first radar beam and a second image from the second radar beam; -   e) co-registering the first and the second images by the SAR data     processor; -   f) comparing, by the SAR data processor, the location of the moving     target in the first image and the second image; -   g) estimating, by the SAR data processor, a velocity of the moving     target based at least in part on the angular offset between the     first and the second radar beams. The angular offset between the     first and the second beam can result in a time offset between the     first and the second image, which can cause a moving target to     appear at different locations in the first and the second image.

FIG. 4 is a block diagram of an example SAR system 400, in accordance with the systems and methods of the present application. SAR system 400 can be a multi-band SAR system, for example a dual-band XL SAR system. SAR system 400 can be on-board a SAR platform such as an aircraft or spacecraft. SAR system 400 comprises a SAR antenna 402, a SAR transceiver 404, a SAR controller 406, a SAR processor 408 (e.g., hardware circuitry), and a communications antenna 410.

SAR antenna 402 can be a shared aperture antenna. SAR antenna 402 can be a planar phased array such as described in International Patent Application Publication WO 2017/044168 entitled “EFFICIENT PLANAR PHASED ARRAY ANTENNA ASSEMBLY”, for example. SAR antenna 402 can be bi-directionally communicatively coupled to SAR transceiver 404. SAR transceiver 404 can be bi-directionally communicatively coupled to data processor 408 and optionally to a data storage (not shown in FIG. 4 ). SAR transceiver 404 can include one or more transceiver circuits, for example operable to transmit pulses and receive returned pulses in respective ones of two or more different frequency bands via one or more antenna such as SAR antenna 402. The transceiver circuits can, for example be commonly housed or on a common circuit board, or housed individually or on respective individual circuit boards. In some implementations, SAR transceiver 404 includes, or consists of, a separate transmitter and receiver, commonly housed or separately housed.

SAR antenna 402 is communicatively coupled to transceiver 404. SAR transceiver 404 can transmit and receive pulses at one or more frequency bands. In some implementations, SAR transceiver is a dual-band SAR transceiver, and can transmit and receive pulses at two frequency bands, for example at X-band and L-band. In some implementations, SAR transceiver 404 can transmit and receive pulses at two or more frequency bands at the same time. The pulses can be synchronized with each other.

SAR transceiver 404 can transmit and receive pulses for one or more imaging modes such as ScanSAR mode and strip-map mode. SAR transceiver 504 can transmit and receive pulses in one or more beams, and in one or more sub-beams. In one example, SAR transceiver 404 transmits and receives L-band pulses in a wide-swath SAR imaging mode, and transmits and receives X-band pulses in a high-resolution imaging mode at the same time (i.e., within the same acquisition window).

SAR controller 406 can comprise one or more processors. SAR controller 406 can include at least one of a Field-Programmable Gate Array (FPGA), an Application Specific Integrated Circuit (ASIC), a microcontroller, and a microprocessor, and one or more programs or firmware stored on one or more nontransitory computer- or processor-readable media.

SAR processor 408 can process SAR data acquired by SAR antenna 402 and SAR transceiver 404. SAR processor 408 can process data in real-time or near-real-time. SAR processor 408 can perform one or more of a variety of processing tasks that may include range compression, azimuth compression, target detection and identification, chip extraction, velocity estimation, and image classification. SAR processor 408 can process data for one or more imaging modes of SAR system 400, for example SAR processor 408 can process one or more of wide-swath ScanSAR mode data, Strip-map mode data, high-resolution Strip-map, and Spotlight mode data.

Communications antenna 410 can transmit and receive data, for example communications antenna 410 can transmit acquired SAR data, processed SAR targets, target detections, identifications, and/or image classifications from SAR system 400 to a ground terminal. Communications antenna 410 can receive commands and/or ancillary data from a ground terminal. The ground terminal (not shown in FIG. 4 ) can include a communications antenna and a transceiver.

FIG. 5 is an isometric view of a SAR antenna in the form of a planar phased array antenna assembly 500, in accordance with the systems and methods described in the present application. The size of planar phased array antenna assembly 500 can be tailored to meet the gain and bandwidth requirements of a particular application. An example application is a dual-band, dual-polarization SAR antenna. A dual-band SAR antenna can operate at L-band and X-band, for example. A dual-polarization SAR antenna can transmit and receive horizontal (H) and vertical (V) polarizations for example.

In an example implementation of a dual-band, dual-polarization SAR antenna, assembly 500 is approximately 2.15 m wide, 1.55 m long and 50 mm deep, and weighs approximately 30 kg. In another implementation, SAR antenna comprises a single panel of dimensions 6 m by 2 m. In yet another implementation, SAR antenna 502 comprises six panels, each panel of dimensions 1 m by 2 m.

While some embodiments described in this document relate to dual X-band and L-band SAR antennas, and the technology is particularly suitable for space-based SAR antennas for reasons described elsewhere in this document, a similar approach can also be adopted for other frequencies, polarizations, configurations, and applications including, but not limited to, single-band and multi-band SAR antennas at different frequencies, and microwave and mm-wave communication antennas.

Antenna assembly 500 comprises a first face sheet 502 on a top surface of antenna assembly 500, containing slots for the L-band and X-band radiating elements. Antenna assembly 500 comprises microwave structure 504 below first face sheet 502. Microwave structure 504 comprises one or more subarrays such as subarray 504-1, each subarray comprising L-band and X-band radiating elements.

Microwave structure 504 can be a metal structure that is self-supporting without a separate structural subassembly. Microwave structure 504 can be machined or fabricated from one or more metal blocks, such as aluminum blocks or blocks of another suitable conductive material. The choice of material for microwave structure 504 determines, at least in part, the losses and therefore the efficiency of the antenna.

Antenna assembly 502 comprises second face sheet 506 below microwave structure 504, second face sheet 506 closing one or more L-band cavities at the back. Second face sheet 506 comprises one or more sub-array face sheets such as 506-1.

Antenna assembly 500 comprises third face sheet 508 below second face sheet 506, third face sheet 508 comprising waveguide terminations. Third face sheet 508 also provides at least partial structural support for antenna assembly 500.

In some implementations, antenna assembly 500 comprises a multi-layer printed circuit board (PCB) (not shown in FIG. 5 ) below third face sheet 508, the PCB housing a corporate feed network for the X-band and L-band radiating elements.

The various embodiments described above can be combined to provide further embodiments. The contents of provisional application U.S. Ser. No. 62/510,182 entitled “SYSTEMS AND METHODS FOR A SYNTHETIC APERTURE RADAR WITH MULTI-APERTURE ANTENNA”, filed on May 23, 2017 and listing as inventors Peter Fox and Stephen Lilley; the contents of International Patent Application Publication WO 2017/044168 entitled “EFFICIENT PLANAR PHASED ARRAY ANTENNA ASSEMBLY”, and the contents of provisional application U.S. Ser. No. 62/510,191 entitled “SYNTHETIC APERTURE RADAR IMAGING APPARATUS AND METHODS FOR MOVING TARGETS”, filed on May 23, 2017 are each incorporated herein by reference in their entirety. Aspects of the embodiments can be modified, if necessary, to employ systems, circuits and concepts of the various patents, applications and publications to provide yet further embodiments.

The foregoing detailed description has set forth various embodiments of the devices and/or processes via the use of block diagrams, schematics, and examples. Insofar as such block diagrams, schematics, and examples contain one or more functions and/or operations, it will be understood by those skilled in the art that each function and/or operation within such block diagrams, flowcharts, or examples can be implemented, individually and/or collectively, by a wide range of hardware, software, firmware, or virtually any combination thereof. In one embodiment, the present subject matter may be implemented via Application Specific Integrated Circuits (ASICs). However, those skilled in the art will recognize that the embodiments disclosed herein, in whole or in part, can be equivalently implemented in standard integrated circuits, as one or more computer programs running on one or more computers (e.g., as one or more programs running on one or more computer systems), as one or more programs running on one or more controllers (e.g., microcontrollers) as one or more programs running on one or more processors (e.g., microprocessors), as firmware, or as virtually any combination thereof, and that designing the circuitry and/or writing the code for the software and or firmware would be well within the skill of one of ordinary skill in the art in light of this disclosure.

While particular elements, embodiments and applications of the present technology have been shown and described, it will be understood, that the technology is not limited thereto since modifications can be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings.

In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure. 

What is claimed is:
 1. A method of operation of a synthetic aperture radar (SAR) system to estimate the velocity of a moving target, the SAR system comprising: a dual-beam SAR antenna, a SAR transceiver and a SAR data processor, the SAR transceiver communicatively coupled to the dual-beam SAR antenna and to the SAR data processor; the method comprising directing a first radar beam to illuminate the moving target in a region on a surface of the Earth by the dual-beam SAR antenna; directing a second radar beam to illuminate the moving target by the dual-beam SAR antenna, the second radar beam at an angular offset relative to the first radar beam; transmitting pulses and receiving backscattered energy simultaneously via the first and second radar beams by the SAR transceiver; generating, by the SAR data processor, a first image from the first radar beam and a second image from the second radar beam; co-registering the first and the second images by the SAR data processor; comparing, by the SAR data processor, the location of the moving target in the first image and the second image; and estimating, by the SAR data processor, a velocity of the moving target based at least in part on the angular offset. 